Ultrasonic matrix array probe with thermally dissipating cable and backing block heat exchange

ABSTRACT

A matrix array probe including a transducer array and integrated circuitry coupled to the transducer elements dissipates heat generated by the array and integrated circuitry through the cover of the transducer probe. A pump in the probe connector pumps fluid through a closed loop system including inbound an outbound fluid conduits in the cable. The fluid conduits in the cable are separated by the cable electrical conductors for the probe. The heat transfer in the probe is done by a heat exchanger in the transducer backing block. Additional cooling may be provided by metal to metal contact with a chiller in the ultrasound system.

This invention relates to medical diagnostic ultrasound systems and, inparticular, to ultrasonic matrix array probes which dissipate heatgenerated by the probe ASIC through the probe cable.

Two dimensional array transducers are used in ultrasonic imaging to scanin three dimensions. Two dimensional arrays have numerous rows andcolumns of transducer elements in both the azimuth and elevationdirections, which would require a large number of cable conductors tocouple signals between the probe and the mainframe ultrasound system. Apreferred technique for minimizing the number of signal conductors inthe probe cable is to perform at least some of the beamforming in theprobe in a microbeamformer ASIC (application specific integratedcircuit.) This technique requires only a relatively few number ofpartially beamformed signals to be coupled to the mainframe ultrasoundsystem, thereby reducing the required number of signal conductors in thecable. However a large number of signal connections must be made betweenthe two dimensional array and the microbeamformer ASIC. An efficient wayto make these connections is to design the transducer array and the ASICto have flip-chip interconnections, whereby conductive pads of thetransducer array are bump bonded directly to corresponding conductivepads of the ASIC.

The high density electronic circuitry of the microbeamformer ASIC can,however, produce a significant amount of heat in its small IC package,which must be dissipated. There are two main directions in which thisheat can flow. One direction is forward through the acoustic stacktoward the lens at the patient-contacting end of the probe. This forwardpath exhibits relatively low resistance to thermal flow. Build-up ofheat in the lens must then be prevented by reducing transmission voltageand/or the pulse repetition frequency, which adversely affects probeperformance.

The preferred thermal conduction direction is to the rear, away from thelens and patient and toward a heat spreader (typically aluminum) at therear of the probe. But generally located behind the transducer stack,the array elements and the microbeamformer ASIC, is an acoustic backingblock. The purpose of the acoustic backing block is to attenuateultrasonic energy emanating from the rear of the acoustic stack andprevent this energy from causing reverberations that are reflectedtoward the acoustic stack. An acoustic backing block is generally madeof a material with good acoustic attenuation properties such as an epoxyloaded with micro-balloons or other sound-deadening particles. Suchmaterials, however, typically have poor thermal conductivity. Hence itis desirable to provide an acoustic backing block for an ultrasoundprobe which exhibits good acoustic attenuation of acoustic energyentering the block, good thermal conductivity toward the rear of theprobe and away from the lens, good mechanical structure which cansupport the acoustic stack as needed, and appropriate electricalisolation of the microbeamformer ASIC from other conductive componentsof the probe.

An acoustic backing block which exhibits these characteristics isdescribed in U.S. patent application Ser. No. 61/453,690, filed Mar. 17,2011. The backing block described in this patent application is formedof a matrix of a highly thermally conductive material with internalacoustic damping members. A preferred material for the thermallyconductive material is graphite exhibiting a high thermal conductivity.The graphite is formed into a rigid block with the mechanical stabilityto support a transducer array stack. The internal acoustic dampingmembers, which can be formed by drilling holes in the graphite blockwhich are filled with acoustic damping material, are preferably locatedsuch that an acoustic wave traveling normal to the rear surface of thetransducer array stack must encounter an acoustic damping member and beacoustically attenuated.

While this thermally conductive backing block is an excellent conductorof heat away from the microbeamformer ASIC, there remains a problem inhow and where to dissipate the heat. Without more, the heat mustdissipate from the probe itself. Ultrasound transducers with no internalelectronics can effectively dissipate heat from the transducer elementsusing modest thermal measures such as metal heat fins in the backing,the probe frame, and heat sinks in the probe. As integrated circuitryhas begun to be located in the probe, passive cooling elements such asheat spreaders internal to and coupled to the probe housing have beenused to dissipate the additional heat. See, e.g., US patent applicationSer. No. 61/486,796, filed May 17, 2011. However, even such improvedpassive cooling is unable to fully dissipate all the heat generated bythe integrated circuitry, leading to compromises in performance such asthose mentioned above to stay below thermal limits. What is needed isadditional capacity to dissipate transducer heat. One further approachbeyond the probe itself is to connect the thermal path in the probe tometal components in the cable (i.e., the signal/power conductors and theshield braid) to dissipate heat through these components. However, thepresent inventors have found that this does not significantly improvethe heat dissipation capability because of limitations in the thermalconductivity along the cable in the conductors and braid, and thethermal conductivity from the conductors and braid to the cable surface,where the heat is dissipated. The present invention is directed to moreeffectively use the transducer cable to help dissipate this additionalheat. This can help manage both lens face temperature and probe handletemperature.

In accordance with the principles of the present invention, anultrasonic matrix array probe is described which dissipates heatgenerated by the probe microbeamformer ASIC through the probe cable bymeans of a fluid based closed loop active cooling system. A heatexchanger in the probe is in thermal communication with a thermallyconductive backing block thermally coupled to the probe ASICs. In oneembodiment the heat exchanger is embedded in the thermally conductivebacking block. A fluid is pumped through fluid conduits in the cable andthrough the probe heat exchanger by a pump located in the probe case orthe probe connector at the proximal end of the cable. The fluid conduitsare formed and arranged in the cable in a manner which efficientlyconducts heat from the fluid to the cable surface where it is dissipatedby radiation and convection. Additional cooling can be provided bymetal-to-metal contact between the probe connector and the ultrasoundsystem which provides additional cooling capacity without a fluidconnection between the ultrasound system and the closed loop system ofthe probe. The closed loop cooling system is completely contained withinthe probe, its cable, and probe connector.

In the drawings:

FIG. 1 illustrates a matrix array probe acoustic stack with a thermallyconductive backing block constructed in accordance with the principlesof the present invention.

FIG. 2 illustrates a matrix array probe, connector and thermallydissipating cable constructed in accordance with the principles of thepresent invention.

FIGS. 3, 4, 5 and 6 illustrate different bundling of transducerconductors and fluid conduits for probe heat dissipation in a probecable in accordance with the principles of the present invention.

FIG. 7 illustrates a heat dissipating probe cable jacket with integrallyformed fluid conduits in accordance with the principles of the presentinvention.

FIG. 8 is a perspective view of a thermally conductive backing block.

FIG. 9 illustrates a thermally conductive backing block with a fluidcooling channel in accordance with the principles of the presentinvention.

FIG. 10 illustrates a thermally conductive graphite foam backing blockconstructed in accordance with the principles of the present invention.

FIG. 11 illustrates an embodiment of the present invention in which heatis exchanged by metal-to-metal contact between a probe connector and achiller in the mainframe ultrasound system.

Referring first to FIG. 1, an acoustic stack 100 with a thermallyconductive backing block which is constructed in accordance with theprinciples of the present invention is shown schematically. Apiezoelectric layer 110 such as PZT and two matching layers 120, 130bonded to the piezoelectric layer are diced by dicing cuts 75 to form anarray 170 of individual transducer elements 175, four of which are seenin FIG. 1. The transducer array 170 may comprise a single row oftransducer elements (a 1-D array) or is a piezoelectric plate diced intwo orthogonal directions to form a two-dimensional (2D) matrix array oftransducer elements. The matrix array 170 may also comprise a one or twodimensional array of micromachined ultrasound transducer (MUTs) formedon a semiconductor substrate by semiconductor processing. The matchinglayers match the acoustic impedance of the piezoelectric material tothat of the body being diagnosed, generally in steps of progressivematching layers. In this example the first matching layer 120 is formedas an electrically conductive graphite composite and the second matchinglayer 130 is formed of a polymer loaded with electrically conductiveparticles. A ground plane 180 is bonded to the top of the secondmatching layer, and is formed as a conductive layer on a film 150 of lowdensity polyethylene (LDPE) 140. The ground plane is electricallycoupled to the transducer elements through the electrically conductivematching layers and is connected to a ground conductor of flex circuit185. The LDPE film 150 forms the third and final matching layer 140 ofthe stack.

Below the transducer elements is an integrated circuit 160, an ASIC,which provides transmit signals for the transducer elements 175 andreceives and processes signals from the elements. Conductive pads on theupper surface of the integrated circuit 160 are electrically coupled toconductive pads on the bottoms of the transducer elements by stud bumps190, which may be formed of solder or conductive epoxy. Signals areprovided to and from the integrated circuit 160 by connections to theflex circuit 185. Below the integrated circuit 160 is a backing block165 which attenuates acoustic energy emanating from the bottom of thetransducer stack. In accordance with the principles of the presentinvention, the backing block also conducts heat generated by theintegrated circuit away from the integrated circuit and the transducerstack and away from the patient-contacting end of the transducer probe.

FIG. 2 illustrates a matrix array transducer probe 14, cable 28 andconnector 32 constructed in accordance with the principles of thepresent invention. The probe components are housed in an externalpolymeric case 20. A strain relief sleeve 18 surrounds the cable 28where it enters the probe case 20. A structure 12 inside the case calleda “spaceframe” supports the internal components of the probe and fits tothe internal dimensions of the case. At the distal, patient-contactingend of the probe is the matrix array acoustic stack 100. Thenon-conductive patient-contacting surface of the probe through whichultrasound waves are sent and received is referred to as a lens 10.Behind the two dimensional transducer array 170 are the beamformer ASICs160 and behind the ASICs and in thermal contact with them is theacoustically attenuating and thermally conductive backing block 165. Inaccordance with the present invention, a heat exchanger 16 is in thermalcontact with the back of the thermally conductive backing block 165.Cooling fluid is pumped into an inlet port of the heat exchanger througha first conduit 22 and warmed fluid carrying heat from the probe flowsout of the probe through a second conduit 24. These conduits passthrough the probe cable 28 along with the signal and power conductorsfor the probe 14. The heat conveyed by the fluid in the second conduit24 dissipates through the external covering of the cable as describedmore fully below. The fluid in this closed loop system is continuouslycirculated by a pump 26 which is coupled to the two fluid conduits 22and 24. The pump is located in the probe connector 32 which connects theprobe and its cable to a mainframe ultrasound system. An electricalsocket 34 in the connector mates with a plug on the ultrasound systemwhen the connector 32 is attached to the ultrasound system. In thisarrangement a cooling fluid is pumped through the cable conduit 22 andthrough the heat exchanger 16, where it picks up heat from the ASICs 160which has been conducted away from the ASICs by the thermally conductivebacking block. The warmed fluid exits the probe 14 through the conduit24 and passes through the cable 28, where it dissipates heat byconvection and conduction through the surface of the cable. Since probecables are quite long, there is a considerable length of cable and cablesurface area from which to dissipate the heat picked up in the probe.The flowing fluid returns in a cooled state to the pump 26 and theprocess continues.

FIGS. 3-6 illustrate a number of techniques for configuring afluid-based, thermally dissipating probe cable 28 for efficient andeffective heat transfer and dissipation from the warmed fluid. In theimplementation of FIG. 3 the outbound (from the probe; heated) conduit24 is located on one side of the cable and the inbound (cool) conduit 22is located on the other side of the cable, separated by the signal andpower electrical conductors running from the connector 32 to the probe14. In this implementation the electrical conductors are bundled intodiscrete sub-bundles 40. By keeping the electrical conductors intosub-bundles the conductor sub-bundles will stay in place around andbetween the fluid conduits and will not separate individually and bedisplaced as can happen with individual unbundled conductors. Thesub-bundles of electrical conductors will thus keep the fluid conduitsseparated from each other thermally on opposite sides of the cable. Thesub-bundles and conduits are surrounded by a metallic and/or graphitecable braid 42 which provides an r.f. electrical shield for theelectrical conductors and also runs the length of the cable. The cablebraid also provides efficient heat transfer from the outbound (warm)conduit 24 to the outer cable jacket. Heat from the fluid in theoutbound conduit 24 is then dissipated from the surface 44 of the cablejacket.

FIG. 4 illustrates another cable configuration in which the inbound(cool) conduit 22 is located in the cable braid 42 with the electricalconductor sub-bundles 40. The other conduit 24 is located outside thecable braid 42. The cable braid shields the conductors as in theprevious example, and the sub-bundles again separate the two conduits.Alternatively the conduits can be reverse-packaged, with the outboundconduit 24 inside the cable braid 42 and the inbound conduit 22 locatedoutside the braid. The electrical conductors, braid and conduits areagain located in the cable jacket 44.

The implementation of FIG. 5 is similar to that of FIG. 3, with theaddition of a thermally conductive layer 46 between the cable braid 42and the outer surface 44 of the cable jacket. The thermally conductivelayer, which may be be a part of the jacket, facilitates efficient heattransfer from the outbound (warm) conduit 24 to the surface of the cable28.

FIG. 6 illustrates another approach to separating the two fluidconduits, which is the use of helically wound tubes 22′, 24′ for the twoconduits. The turns of the two helical windings are in interleavedalteration so that the two fluid conduits are always separated. Theelectrical conductors run through the center of the two helical tubesand the cable jacket 44 encases the helical conduits and conductors. Ahelically wound conduit presents a greater surface area to the cablejacket than does a straight conduit, affording greater heat transferfrom the warmed fluid inside the conduit and cable.

In the implementation of FIG. 7 the fluid conduits 22, 24 are formedintegral with the cable jacket 44. The fluid conduits 22, 24 are thusformed during the extrusion of the cable jacket. The two fluid conduitswill stay separated because they are integrally attached to oppositesides of the jacket, with the shielded electrical conductors runningthrough the center of the jacket.

There are several ways to implement effective heat exchange between thethermally conductive backing block 165 and the heat exchanger 16 in theprobe. One is to form the heat exchanger 16 as part of the spaceframe ofthe probe. The spaceframe is generally made of aluminum, which is anefficient conductor of heat. The heat exchanger 16 in FIG. 2 is then across-member of the spaceframe which mounts the acoustic stack and itsthermally conductive backing block, with the backing block in thermalcommunication with the cross-member 16. A number of fluid channels aremachined through the cross-member 16 and coupled to the fluid conduitsso that incoming fluid from conduit 22 flows through the channels andout through the outbound conduit 24. As the cross-member 16 is heated byheat transferred to it by the backing block, that heat is carried awayby the fluid flowing through the channels of the cross-member.

Another heat exchange implementation in the probe is to include aPeltier device in the heat exchanger 16, in thermal communication withthe thermally conductive backing block 165. A Peltier device has ametal-to-metal junction of two types of metal. When an electricalcurrent is applied to the junction, one side gets cool and the otherside gets warm. With the cool side in thermal contact with the backingblock, the Peltier device will then draw heat out of the backing block.A fluid tube, coil, or channel member of a heat exchanger as previouslydescribed, through which fluid of the fluid conduits 22, 24 flows, is inthermal communication with the warm side of the Peltier device andconducts and carries away heat from the warm side of the device throughthe fluid of the outbound conduit 24.

A third probe heat exchange implementation is shown in FIGS. 8, 9, and10. In this implementation fluid heat exchange is done within thethermally conductive backing block. FIG. 8 illustrates a thermallyconductive and acoustically attenuating graphite backing block asdescribed in aforementioned U.S. patent application Ser. No. 61/453,690,filed Mar. 17, 2011. In this illustration the graphite is renderedtranslucent for clarity of illustration of the internal compositestructure of the block. The acoustic dampening members are formed as aplurality of angled cylinders 30 of backing material in the backingblock. The cylinders 30 are cut or drilled into the graphite block 20,then filled with acoustic dampening material such as epoxy filled withmicro balloons or other acoustic damping particles. The tops of thecylinders 30 present a large area of acoustic dampening material to theback of the integrated circuit 160. A considerable amount of theundesired acoustic energy emanating from the back of the integratedcircuit and acoustic stack will thus pass immediately into the dampeningmaterial. The angling of the cylinders as seen in FIG. 8 and thecross-sectional view of FIG. 9 assures that acoustic energy traveling inthe Z-axis direction away from the ASIC will have to intersect dampeningmaterial at some point in the path of travel. Preferably, there is nopath in the Z-axis direction formed entirely of graphite, and theangling of the cylinders does not promote reflection of energy back tothe integrated circuit but provides scattering angles downward and awayfrom the integrated circuit. In practice it may be sufficient to blockmost of the Z-axis pathways such as by blocking 95% of the pathways.Thus, the angling of the cylinders assures damping of all orsubstantially all of the Z-axis directed acoustic energy.

Heat, however, will find continuous pathways through the graphitebetween the cylinders 30. Since the flow of heat is from highertemperature regions to lower (greater thermal density to lesser), heatwill flow away from the integrated circuit 160 and acoustic stack 100 tostructures below the backing block 165 where it may be safelydissipated.

For fluid-based closed loop cooling a fluid channel 54 is formed in thebacking block 165 as shown in the cross-sectional view of FIG. 9. Theinbound (cooling) conduit 22 is coupled to the inlet port 52 of thefluid channel 54 and the outbound (warm) conduit 24 is coupled to theoutlet port 56 of the fluid channel. As heat is transferred into thebacking block 165 from ASIC 160, it is carried away from the backingblock and probe by the fluid flow through the outbound fluid conduit 24.Heat exchange is done within the backing block itself with no separateheat exchanger.

Another implementation of this technique is shown in FIG. 10. In thisimplementation the thermally conductive and acoustically attenuatingbacking block 165 is formed of highly porous graphite foam 36. A coatingof epoxy resin 38 on the outer surface of the graphite foam blockprovides structural rigidity and an epoxy surface that readily bonds tothe ASIC 160. Holes are drilled through the epoxy layer on either sideof the block and fluid ports 52 and 56 are located in the holes. Thefluid ports thus access the highly porous interior of the block 165. Theopen structure of the porous graphite foam allows fluid to flow from oneport to the other. Cool fluid flows in one port from the inbound conduit22, through the porous foam structure, and out through the other portand into the outbound conduit 24. The graphite 36 efficiently conductsheat into the block 165 to be carried away by the fluid flow, the openfoam structure promotes the fluid flow, and the graphite particleseffectively scatter and attenuate acoustic energy from the back of theacoustic stack.

If even greater heat dissipation is required than that afforded throughthe cable 28, additional cooling can be provided from the ultrasoundsystem. Preferably, this additional cooling is provided without anyfluid communication between the connector and the ultrasound system; itis desirable to keep the closed loop fluid flow completely within theprobe, cable and connector. In FIG. 11 a metal plate 60 with fluidchannels passing therethrough is located in the connector with theelectrical socket 34. The outbound (warm) fluid conduit is coupled toone end of the fluid channels of plate 60, and a fluid conduit 23 iscoupled from the other end of the fluid channels to the pump 26. Warmfluid will thus flow through the plate 60, heating the plate, beforebeing pumped back to the cable and probe. When the probe connector 32 isplugged into the ultrasound system 200 and the electrical socket mateswith the matching plug 34′ of the ultrasound system, the plate 60 ispressed into contact against another fluid channel plate 62 of theultrasound system. The plate 62 is cooled by apparatus in the ultrasoundsystem. Since space and power is not at a premium in the ultrasoundsystem as it is in the probe, a cooling system of almost any design canbe used in the ultrasound system. A preferred cooling system is achiller/evaporator 68, located in and powered by the ultrasound system,which pumps chilled fluid through fluid conduits 64, 66 and fluidchannels of the plate 62. The plate 62 is thereby chilled to aconsiderably low temperature relative to ambient temperature. The metalto metal contact of chilled plate 62 and connector plate 60 which iswarmed by fluid from the probe effects a rapid and efficient transfer ofheat from the fluid in the warmed plate 60 to the chilled plate 62. Thethermal communication between the two plates is established when theprobe connector is plugged into the ultrasound system, and no fluidpasses between the probe components and the ultrasound system.

Variations of the systems described above will readily occur to thoseskilled in the art. A heat exchanger need not be made of metal elements,but can use other conductive elements such as graphite, silicon, orother conductive materials. Obstruction of the fluid conduits due tobending, kinking, or twisting of the cable can be minimized by usingredundant conduits, such as two inbound conduits alternated with twooutbound conduits and located every 90° around the cable. Flowmonitoring can be employed to assure the continued operation of thecooling system through the use of flow sensors, pressure sensors, ortemperature monitoring. A fluid reservoir can be connected to the fluidloop to provide for expansion and contraction of the fluid due totemperature and pressure changes.

1. An ultrasonic transducer probe assembly comprising: a probe case; anarray of transducer elements located behind an acoustic window in theprobe case and configured to transmit and receive ultrasound energythrough the acoustic window; an integrated circuit coupled to thetransducer elements in the probe case and configured to process signalstransmitted or received by the array; a thermally conductive backingblock thermally and acoustically coupled to the array of transducerelements and the integrated circuit which attenuates or scattersultrasound energy received from the transducer elements, the thermallyconductive backing block comprising a porous structure; a probeconnector configured to connect the transducer probe to an ultrasoundsystem; a cable connected between the probe case and the probe connectorand containing a plurality of electrical conductors coupled between theprobe case and the probe connector; and a fluid-based closed loopcooling system comprising: a closed fluid loop extending through thecable from the probe case to the probe connector; a pump, coupled to thefluid loop, which pumps fluid through the loop; and a heat exchangerwhich acquires heat generated by the transducer array and integratedcircuit for dissipation outside the closed fluid loop, wherein the heatexchanger comprises a fluid channel that is coupled to the closed fluidloop and is configured to allow fluid to pass through the porousstructure of the thermally conductive backing block.
 2. The ultrasonictransducer probe assembly of claim 1, wherein the closed fluid loopfurther comprises an inbound fluid conduit and an outbound fluidconduit, wherein the inbound and outbound fluid conduits are coupled tothe fluid channel of the thermally conductive backing block.
 3. Theultrasonic transducer probe assembly of claim 2, further comprisingfirst and second fluid ports, coupled to the fluid channel of thethermally conductive backing block, wherein the inbound fluid conduit iscoupled to the first fluid port and the outbound fluid conduit iscoupled to the second fluid port.
 4. The ultrasonic transducer probeassembly of claim 1, wherein the thermally conductive backing block isformed of a material having a thermal conductivity at least as great asthat of graphite and comprises a composite structure of acousticdampening material located in the thermally conductive backing block. 5.The ultrasonic transducer probe assembly of claim 4, wherein thecomposite structure comprises a plurality of holes in the backing blockwhich are filled with acoustic dampening material.
 6. The ultrasonictransducer probe assembly of claim 5, wherein the holes further comprisea plurality of cylindrical holes formed in the backing block andextending from a top surface of the backing block to a bottom surface ofthe backing block, wherein the acoustic dampening material in the holesat the top surface comprise a majority of an area of the top surface. 7.The ultrasonic transducer array assembly of claim 6, wherein the holesare angled at a non-parallel angle with respect to a direction normal toa plane including the array of transducer elements.
 8. The ultrasonictransducer array assembly of claim 7, wherein the holes are angled at anangle which causes acoustic energy traveling in the direction to bescattered away from the array of transducer elements.
 9. The ultrasonictransducer array assembly of claim 4, wherein the thermally conductivebacking block is formed of graphite.
 10. The ultrasonic transducer arrayassembly of claim 1, wherein the thermally conductive backing block isformed of a composite foam backing block, located rearward of the arrayof transducer elements and integrated circuit in the probe case, thecomposite foam backing block being formed of a foam material having aporous structure; and an epoxy resin filling at least some of the porousstructure of the foam backing block.
 11. The ultrasonic transducer arrayassembly of claim 10, wherein the foam material further comprises agraphite foam.
 12. The ultrasonic transducer array assembly of claim 10,wherein the composite foam backing block further comprises an exteriorsurface, and wherein the epoxy resin fills the porous structure of thefoam backing block adjacent to the exterior surface.
 13. The ultrasonictransducer array assembly of claim 12, further comprising first andsecond fluid ports, coupled to the fluid channel of the composite foambacking block, wherein the fluid port extend through the exteriorportion of the foam backing block which is filled with epoxy resin, andwherein the inbound fluid conduit is coupled to the first fluid port andthe outbound fluid conduit is coupled to the second fluid port.
 14. Theultrasonic transducer array assembly of claim 10, wherein the integratedcircuit further comprises a beamformer ASIC coupled to a rearward sideof the array of transducer elements, wherein the composite foam backingblock is thermally coupled to the beamformer ASIC.
 15. The ultrasonictransducer array assembly of claim 14, wherein the composite foambacking block is bonded to the beamformer ASIC by an epoxy bond.